The invention relates to a catalyst subassembly for a device for purifying exhaust gases from an internal combustion engine, in particular a diesel engine, having a particulate filter that can be regenerated and an integral catalyst on the particulate filter for selective reduction of nitrogen oxides (SCRF catalyst). The invention furthermore relates to a modular system for the catalyst subassembly, to a device containing the catalyst subassembly and to a method for producing the catalyst subassembly.
To purify the exhaust gases from modern motor vehicles, especially diesel vehicles, use is generally made of catalyst arrangements comprising a combination of an oxidation catalyst or an NOx storage catalyst and a particulate filter. The close-coupled oxidation catalysts and particulate filters are generally installed in a common housing. To comply with stricter exhaust limits, the oxidation catalyst can be replaced by an NOx storage catalyst within the same overall volume.
In the oxidation catalyst, carbon monoxide and volatile hydrocarbons are oxidized to form carbon dioxide. The oxidation catalyst generally consists of a metallic or ceramic supporting substrate having a catalytically active coating composed of metals from the platinum group, such as platinum and/or palladium. Since the exhaust gas temperatures of a diesel engine are significantly lower than the exhaust gas temperatures of a spark ignition engine, oxidation catalysts for diesel engines (DOC) are preferably installed close to the exhaust manifold.
Wall flow filters and bypass flow filters can be used as particulate filters that can be regenerated. In the case of wall flow filters, soot particles and other solids present in the exhaust gas flow are fixed on a porous filter wall by adhesion. When the exhaust gas backpressure reaches a predetermined threshold value owing to particulate deposits, regeneration of the particulate filter is initiated. For this purpose, an afterinjection of fuel into the exhaust gas flow can be carried out, for example, in order to increase the exhaust gas temperature and burn the soot particles deposited in the filter. In the case of the bypass flow filter technique, some of the exhaust gas flow is diverted and passed through a nonwoven layer. During this process, the soot particles are filtered out. In the case of bypass flow filters, continuous regeneration is generally performed.
For heavy vehicles, the catalyst arrangement consisting of an oxidation catalyst and a particulate filter can be supplemented by an SCR catalyst. Here, SCR (selective catalytic reduction) refers to the selective catalytic reduction of nitrogen oxides (NOx) from exhaust gases of internal combustion engines. An SCR catalyst of this kind is described in EP 1 203 611 B1 or EP 1 837 494 B1, for example.
To operate the SCR catalyst, a reducing agent for the nitrogen oxides, e.g. an aqueous urea solution, which can be obtained commercially in the form of “AdBlue™”, is sprayed into the exhaust gas flow upstream of the catalyst. In the SCR catalyst, hydrolysis of the urea solution to form ammonia is performed in a first step, and conversion of ammonia and NOx to water and nitrogen is performed in a second step.
DE 103 23 607 A1, WO 2005/016497 and U.S. 2011/0078997 each disclose catalysts which have a combination of an SCR catalyst and a particulate filter on a single substrate. These catalysts are generally referred to as SCRF catalysts (SCR on filter) and can both fix soot particles and reduce nitrogen oxides NOx to form nitrogen.
DE 10 2012 209 852 A1 shows a catalyst arrangement in which an SCRF system is supplemented by a downstream SCR catalyst, wherein the SCR catalyst has an additional storage function for hydrocarbons. An oxidation catalyst, which can oxidize the hydrocarbons and carbon monoxide, is arranged downstream of the SCR catalyst.
DE 10 2009 038 835 A1 discloses an exhaust gas purification system which has a first SCR catalyst upstream of an oxidation catalyst and a particulate filter and has a further SCR catalyst downstream of the particulate filter.
DE 10 2011 079 785 discloses an exhaust system in which a first oxidation catalyst (DOC) is provided, which is followed in the direction of flow of the exhaust gas by an SCR unit and a second DOC. This system is intended to ensure lower hydrocarbon emissions. The SCR unit can be supplemented by a SCRF catalyst positioned after said SCR unit.
The catalyst arrangements mentioned above are designed to comply with the ever stricter limits for the nitrogen oxide and hydrocarbon content of exhaust gases from internal combustion engines.
However, one disadvantage of close-coupled SCRF catalysts is that the component volume of the SCRF catalyst is greater than the previously used particulate filters coated with noble metal, also referred to as “Coated Soot Filters” (CSF). In the past, design parameters for the CSF included the “ash accumulation capacity” and the maximum exhaust gas mass flow of the internal combustion engine. In addition, the NOx conversion of the SCRF catalyst must also be taken into account, leading to a larger component volume. However, the larger component volume of the SCRF catalyst entails an increased weight, higher costs and greater thermal inertia.
It is the object of the present invention to provide an exhaust gas purification system which is reliable in all operating situations, which can be manufactured at low cost and with which the above disadvantages can be avoided.
These and other objects are achieved by means of a catalyst subassembly for a device for purifying exhaust gases from an internal combustion engine, in particular a diesel engine, in accordance with embodiments of the invention.
According to the invention, the catalyst subassembly includes a particulate filter that can be regenerated and an integral catalyst on the particulate filter for selective reduction of nitrogen oxides (SCRF catalyst). A catalyst for selective reduction of nitrogen oxides (SCR catalyst) is provided upstream of the SCRF catalyst, wherein the SCRF catalyst and the SCR catalyst are arranged in a common catalyst housing.
Thus, an aspect of the invention is a hybrid construction which includes an SCR catalyst on a dedicated supporting substrate and an SCRF catalyst positioned after said SCR catalyst and having a component volume which corresponds to the volume of a CSF customary for the respective variant of the internal combustion engine. This hybrid construction has the advantage that a modular system for different vehicles can be provided, in which the SCRF catalysts are of identical dimensions and the dimensions of the SCR catalysts are varied, depending on the vehicle size or the engine variant. The catalyst subassembly can therefore be produced at lower cost.
Further advantages of the catalyst subassembly according to the invention are obtained by virtue of the fact that the SCR catalyst exhibits a more rapid light off behavior of the SCR function in normal operation and better NOx conversion in the emissions cycle by virtue of its lower thermal inertia. This results in improved efficiency of the catalyst subassembly for the same overall volume. The smaller volume of the particulate filter furthermore results in a lower overall weight. Moreover, the costs of the hybrid system are lower since the SCR catalyst can be manufactured more advantageously than a particulate filter. In the regeneration mode of the particulate filter, the subassembly can heat up more quickly because of the lower heat capacity of the SCR filter.
According to a preferred embodiment, the SCR catalyst includes a metallic or ceramic through-flow substrate. The through-flow substrate is typically a honeycomb monolith with a series of elongate channels which extend through the monolith, wherein the channels are open at both ends. A through-flow substrate of this kind can be manufactured with different axial extents at low cost. In this way, the NOx conversion of the SCR catalyst can be matched easily to different engine variants. The differences in the various axial extents are greater here than the manufacturing tolerances which generally occur in the manufacture of substrate monoliths for catalysts.
The SCRF catalyst is preferably formed as a filtering monolith, particularly preferably as a wall flow filter, e.g. in the form of a ceramic porous filter substrate which has a multiplicity of inlet channels arranged parallel to one another and a multiplicity of outlet channels. Each inlet channel and each outlet channel is partially defined by a ceramic wall of a porous structure. In this arrangement, each inlet channel is alternately separated from an outlet channel and vice versa by a ceramic wall of a porous structure. In other words, the wall flow filter is a honeycomb arrangement which defines a multiplicity of first channels closed at the upstream end and a multiplicity of second channels, which are not closed at the upstream end but are closed at the downstream end. Channels which are vertically and laterally adjacent to a first channel are closed at the downstream end. When viewed from one of the ends, the alternately closed and open ends of the channels have the appearance of a chessboard.
The SCRF catalyst can be designed as a full extrudate or as a coated supporting substrate. Suitable catalytically active substances include mixed oxides of vanadium and titanium, iron- and copper-containing zeolites and mixtures thereof.
The through-flow substrate of the SCR catalyst is preferably a disk-shaped honeycomb monolith, in particular one which has a cross section which is substantially cylindrical or polygonal, preferably rectangular. The geometry of the through-flow substrate is preferably matched to the external dimensions of the SCRF catalyst.
As a catalytically active component for the SCR catalyst, the conventional coatings and full extrudates known in the prior art can be used. Examples of suitable catalysts for selective reduction of nitrogen oxides are copper-containing compounds, mixed oxides and oxide mixtures of vanadium oxide and titanium dioxide and optionally tungsten oxide, molybdenum oxide and silicon dioxide, iron- and copper-containing zeolites and acidic zeolites, which can be partially exchanged with metal ions, and mixtures thereof.
As a particularly preferred option, at least one spacer element is provided between the SCR catalyst and the SCRF catalyst or an axial wall surface of the catalyst housing. By use of the spacer elements, the different axial extents of the SCR catalyst can be compensated while keeping the volume of the catalyst housing and of the SRCF catalyst constant. Solid spacers and/or nonwovens, knitted fabrics and woven fabrics can be used as spacer elements, these also generally being used to compensate manufacturing tolerances in the catalyst substrates. Heat-resistant wire meshes are preferred.
The invention furthermore relates to a modular system for the catalyst subassembly. The modular system according to the invention serves for use for a plurality of variants of an internal combustion engine, in particular a diesel engine, and includes a plurality of catalyst housings and SCRF catalysts, which are each dimensioned for the plurality of variants of the internal combustion engine independently of the engine variant, and a set of different SCR catalysts, which are each dimensioned for the plurality of variants of the internal combustion engine in a manner dependent on the engine variant and have different outside dimensions.
The SCRF catalysts and the catalyst housings of the modular system preferably each have identical external dimensions, which are suitable for use with a plurality of engine variants. The catalyst housings and SCRF catalysts in the module can preferably each have an identical volume and/or an identical ratio of length to cross section.
Here, identical external dimensions or an identical volume or an identical ratio of length to cross section means that the dimensions of the SCRF catalysts and/or of the catalyst housings in the module fluctuate only within the normal manufacturing tolerances. Matching to a plurality of engine variants is accomplished by optimizing and defining the dimensions or dimensioning of the catalyst housing and of the SCRF catalyst, thus achieving a sufficient reduction in pollutants while complying with predetermined limits for all engine variants of the module without an additional SCR catalyst.
Dimensions of the plurality of SCR catalysts in the modular system are preferably chosen so that the respective SCR catalyst has an axial extent matched to the associated engine variant and optionally the dimensions of the SCRF catalyst. By selecting the SCR catalyst with the axial extent that matches the respective engine variant, the nitrogen oxides in the exhaust gas flow, in particular, can then be lowered to values below the legal limits.
In addition, the modular system can include a set of different spacer elements, which are matched to the dimensioning of the plurality of SCR catalysts. Since differently dimensioned SCR catalysts are used, depending on the engine variant, it is possible in this way to compensate any empty spaces that may arise in the catalyst housing. The spacer elements can be solid spacers, such as spacing rings, or the heat-resistant wire meshes that are also used to compensate manufacturing tolerances can be used with different wall thicknesses.
The invention furthermore comprises a device for purifying exhaust gases from an internal combustion engine having the catalyst subassembly according to the invention.
In particular, the device can contain a unit for supplying a reducing agent, in particular ammonia or an ammonia precursor, which is arranged upstream of the catalyst subassembly. The reducing agent is preferably a urea solution that can be obtained commercially in the form of “AdBlue™”, the composition of which is internationally standardized. A mixing element or an evaporator, which brings about uniform distribution of the reducing agent in the exhaust gas flow, can be provided downstream of the unit for supplying the reducing agent and ahead of the catalyst subassembly.
Moreover, the device can have a known oxidation catalyst and/or an NOx storage catalyst, which is/are in each case arranged in the exhaust tract upstream of the subassembly and of the unit for supplying a reducing agent.
As a further preferred embodiment, the device can contain an ammonia barrier catalyst which is positioned after the subassembly consisting of the SCR catalyst and the SCRF catalyst and arranged downstream of the subassembly. The ammonia barrier catalyst prevents the troublesome smells due to “ammonia slip”, which can be caused by overmetering of the reducing agent.
The method according to the invention for producing the catalyst subassembly from a modular system for a plurality of variants of an internal combustion engine, in particular a diesel engine, comprises the following acts:                a) providing a plurality of catalyst housings and a plurality of SCRF catalysts, each having identical dimensions;        b) selecting an SCR catalyst dimensioned in accordance with the respective engine variant from a set of different SCR catalysts; and        c) assembling the selected SCR catalyst with the SCRF catalyst in the catalyst housing to form the catalyst subassembly.        
By way of the method according to the invention, a modular construction of the catalyst subassembly can be achieved. Since only the low-cost SCR catalyst is varied, both the inventory costs and the manufacturing costs of the subassembly can be reduced overall.
According to another embodiment of the method, the module additionally contains a set of differently dimensioned compensating elements, wherein at least one compensating element for the respective engine variant is selected from the set of compensating elements in accordance with the dimensioning of the SCR catalyst and installed in the catalyst housing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.